For the past years we have been analyzing the mechanism(s) active in the immunoregulation of the developing and involuting granulomas which form around parasite eggs in Schistosoma mansoni infected mice. Our studies indicate that by lymphokine-secreting T lymphocytes which belong to the Lyt 1 plus Ia minus subpopulation. At the chronic stage of the infection the granulomatous response undergoes a spontaneous diminution (modulation). This occurs via the activity of T suppressor lymphocytes which belong to a different (Lyt 2.3 plus Ia plus subpopulation. Suppression of the granulomatous response occurs via the interaction between lymphokine secreting and regulatory T lymphocytes with resultant suppression of lymphokine production. Recent observations indicate, that the immunomodulated granulomas are not only smaller in size, but also contain macrophages which have an impaired phagocytic capacity to ingest opsonized red cells. The impairment is connected with Fc and c3 receptor function.